1. Field of the Invention
The present invention relates to a varioscope optical unit and to a microscope having such an optical unit.
2. Description of the Related Art
It is a requirement for many microscopy applications that the observation object that is observed using a microscope remains in the focus of the microscope when the working distance of the microscope from the observation object, also referred to as depth of field, changes. In order to achieve this, the focal length of the objective optical unit must be variable so as to be able to be adapted to the change in the working distance. In so-called varioscopes, this is achieved by way of an optical arrangement having a lens or lens group with a positive refractive power and a lens or lens group that is arranged on the object-side and has a negative refractive power, with one of the lenses or lens groups being arranged such that it is displaceable along the optical axis. A stereo microscope having a corresponding optical arrangement, in which the lens having a negative refractive power is arranged such that it is displaceable along the optical axis, is described, for example, in JP 11271625 A. In the arrangement described in JP 11271625 A, illumination light is coupled in below the negative member via a deflection mirror.
If the illumination radiation in a varioscope optical unit is intended to be coupled into the microscope beam path above the positive member, the lenses or lens groups of the varioscope optical unit have cutouts for the illumination radiation path, so that back-reflections of the illumination into the observer-side pupils of the microscope are avoided. However, Lenses and lens groups that have cutouts are comparatively expensive and generally permit illumination only under a relatively large illumination angle.
It is therefore an object of the present invention to provide a varioscope optical unit, in which a low-reflection illumination can be realized when coupling in the illumination light above the positive member, even without using lenses or a lens group with cutouts. A further object is to provide a microscope in which disturbing reflections are reduced for an illumination beam path that extends through the varioscope optical unit.